In conventional devices in information communication systems, a grating coupler that is provided along the optical waveguide is used in order to input signal light that has propagated through an optical fiber into an optical waveguide and guide the signal light through the optical waveguide. Here, an optical coupling structure using a conventional grating coupler is described in reference to FIGS. 19A and 19B.
FIGS. 19A and 19B are diagrams illustrating a conventional coupling state between an optical fiber and a grating coupler. FIG. 19A is a perspective diagram, and FIG. 19B is a cross-sectional diagram along the single-dotted chain line A-A′ in FIG. 19A. The grating coupler has such a structure that light that is propagating through an optical waveguide 91 is radiated upward or downward from the grating (diffraction grating) 95 that is provided in a grating coupler unit 94 so that the optical waveguide 91 is coupled with an external optical fiber 96. Conversely, the grating coupler is also used for the coupling of light that propagates in the opposite direction, that is, from the optical fiber to the optical waveguide. Since this is a plane structure, it can be easily formed in accordance with a photolithographic technology, and in particular, this has become a mainstream optical fiber interface in silicon photonics. Here, 92, 93, 97 and 98 in FIGS. 19A and 19B are a lower clad layer, an upper clad layer, a core and a clad, respectively.
However, a coupling efficiency of 100% cannot theoretically be expected for grating couplers that have such a basic structure without any modifications. There are some reasons for this, and one of the main reasons is a mismatch in the form of the beam. FIG. 20 is a graph illustrating the form of a beam radiated from a grating coupler, and as shown in the graph, the optical fiber has a profile as that of a Gaussian beam. Meanwhile, the form of the beam radiated from the grating coupler has a profile in an exponential function form where light gradually attenuates.
This mismatch generates a loss of approximately 1 dB even in the best state. In addition, a further greater loss is generated when the state is shifted from the optimal state due to the manufacturing tolerance. This issue has been known for a long time, and a fundamental solution is to allow the grating coupler toradiatea Gaussian beam.
In order to convert the exponential function beam to a Gaussian beam, the coupling between the grating and the optical waveguide may be gradually intensified toward the end terminal of the grating coupler. A method for radiation a Gaussian beam by changing the line width of a grating has been proposed, for example (see Patent Literature 1 or Non-Patent Literature 1). As can be easily analogized from FIG. 20, however, a Gaussian beam cannot be radiated unless the portion with the highest light intensity is converted to the portion with the lowest light intensity. This means that a very fine line width is required.